OBJECTIVES: (a) The comparative uptake of tracer amounts of metals of varying neurotoxicity (inorganic Pb, Ng and Cd) has been examined. Substantially different patterns of distribution of these compounds in the Brain Barrier System (BBS) have been found. In particular the uptake of Cd (the least neurotoxic metal) by the choroid plexus is found to be the most concentrative and highly regulated. It is possible that this selective sequastiation of the metal may subsequently alter general neural distribution of the compound and so modify its brain toxicity. (b) We have attempted to assess processes underlying short-term Hg accumulation in various tissues in the adult rat. IV injections of HgCl2 have been given either alone or in combination with L-cysteine and D-cysteine. The results suggest that different processes underlie short-term uptake of the metal by liver, brain and erythrocytes. They also indicate that direct complexation of the metal by either thiol may be a critical factor in the control of Hg deposition in these tissues and that the extent of this complexation may be different for liver, brain and erythrocyte. The results advance the aims of the study by suggesting an approach to greater chemical definition of the BBS sites for metal uptake. (c) The patterns of uptake for inorganic Pb by the developing albino rabbit at different ages have been observed. The results suggest that the neural "barrier" to inorganic lead appears established by 3 days of age in the albino rabbit. The barrier remains constant for at least 30 days and is uninfluenced by substantial changes in circulating and extraneural tissue levels. The specific translocation systems underlying this "barrier" effect may therefore develop in the prenatal period.